Probably pretty popular question.
I have laptop with Core 2 Duo and 2 gb ram. What better to install?
Amd64 or i386? Use SMP or not? It will be very plain desktop with tiling window manager, some office work and so on. I don't need wine (if i will need some old windows games i probably will install linux/wine in qemu and play "there") or flash.

Qemu is plain broken on AMD64! If you are planning to use Qemu the way to go is i386.
Actually Qemu without KQemu accelerator is useless. Kqemu exists only for i386. Even with kqemu idea of running Linux in the virtual machine so that you can use Flash is plain silly. If you just need occasional Flash you are far better of to go with LiveCD. I really like SLAX because it enables you to customize your LiveCD.

What else. Wine doesn't work on OpenBSD because it has bugs and OpenBSD is unforgiving.
For most people it really doesn't matter if they are going to use SMP or not. OpenBSD still has a
big Giant Lock in the kernel and it is not optimized for SMP. Also it uses n:1 threading model.
In order for an application to take full advantage of multi core machine application and OS has to be written for n:m threading model. An example is MySQL. An example of application which runs equally
fast if not better on single core is PostgreSQL.

OpenBSD amd64 is fully 64 bit operating system unlike FreeBSD and NetBSD counter parts. That might even mean penalization in the terms of performance as most desktop applications are still written for 32 bit machines. Also application will have to be compiled on 64 bit platform since OpenBSD amd64 does NOT run and will never run i386 binaries.

Above being said I run amd64 on machine DeLL Optiplex with Core 2 duo and 4 GB of RAM at work
as my personal workstation and I also run i386 at home on home made Core 2 duo with 3 GB of RAM. The only thing I like about i386 that I can run Opera via Linux emulation. That is about it. There is absolutely zero difference in performance. In both cases bottle neck is slow Network and poor HDD performance of IDE (SATA) HDD. In both cases I run MP kernel. I find it usefull when ripping and
encoding DVDs for my kids. Then MP kernel really makes a difference.

Bigmem is not production ready on OpenBSD so if you will not be able to access more than 3.4
GB of RAM anyway. It doesn't matter because as I said in the best case scenario (you have really high network band width) your bottle neck is slow HDD. You will need SCSI HDD to see the difference.

That is from 2008. The variable bigmem controls it, in /usr/src/sys/arch/amd64/amd64/machdep.c and is disabled (set to zero) by default due to stability problems addressing more than 3GB of RAM. There have been many threads, one of them starts here: